By virtue of improved storage capacity, more frequent rechargebility and higher energy densities, batteries can be used in ever broader applications. Batteries having a relatively low energy storage capacity are used, for example, for small portable electronic appliances such as mobile telephones, laptops, camcorders or the like, while batteries with a high capacity are used as energy source for the drive of motors of hybrid or electric vehicles, etc. or as stationary batteries.
Batteries can be formed, for example, by connecting battery modules to one another in series, wherein sometimes the battery modules are also connected to one another in parallel and the battery modules for their part can comprise battery cells which are connected in series and/or in parallel.
For driving motors of hybrid or electric vehicles, in particular battery module strings are suitable, which are also referred to as battery converters (BDCs). Battery module strings comprise at least two battery modules which are connected in series, wherein further battery modules can be connected in parallel. In this case, the battery modules have a coupling unit and are individually connectable and disconnectable with the aid of said coupling unit. Therefore, the battery module string can be used, by virtue of corresponding connection and disconnection of the modules, to generate an oscillating voltage characteristic. With a corresponding configuration, a voltage profile with an approximately sinusoidal characteristic which can be used for driving electric or hybrid motors can be generated, for example.
Battery management systems are used for battery management, for example for basic actuation of modules, for increasing the safety of batteries, for increasing efficiency and for extending the life of battery modules and battery systems comprising battery modules. One task of battery management systems consists in the determination of the present state of the battery modules. The important information for this purpose includes the impedance, the internal ohmic resistance of the battery module or of the battery cells contained in the battery module, wherein the impedance is dependent on the state of charge, the temperature and the degree of aging of the battery cells.
In accordance with the prior art, associated measurements are performed outside normal operation of the battery. For example, ISO 12405 proposes applying pulse-shaped charging and discharging currents to batteries to be tested in a test environment for time periods in the seconds range and measuring the voltage across the battery prior to and after the current pulse. The impedance then results as a ratio of the difference of the measured voltages to the intensity of the current pulse.
Since the current pulses are present for one or more seconds, the two voltage measurements are also apart from one another by this time period.